بُلُوغُ الْمَرَامِ
Bulugh al-Maram 155
The Book of Prayer
وَعِنْدَهُمَا مِنْ حَدِيثِ جَابِرٍ: { وَالْعِشَاءَ أَحْيَانًا وَأَحْيَانًا: إِذَا رَآهُمْ اِجْتَمَعُوا عَجَّلَ, وَإِذَا رَآهُمْ أَبْطَئُوا أَخَّرَ, وَالصُّبْحَ: كَانَ اَلنَّبِيَّ - صلى الله عليه وسلم -يُصَلِّيهَا بِغَلَسٍ } [1] .
Narrated Jabir (RA), in the version of al-BUkhari and Muslim: [The Prophet (ﷺ)] used to advance the 'Isha' prayer sometimes and delay it at other times. Whenever he saw the people assembled (for the 'Isha' prayer) he would pray early and if the people delayed, he would delay the prayer. And the Prophet (ﷺ) used to offer the Fajr prayer when it was still dark.
Sunnah.com reference: Book 2, Hadith 5